How can I retrieve my trusted number on iCloud if I forgot it?

When you choose Can't Get to my iPhone, you should see a partially redacted phone number that is the Trusted Phone number that has been used on your Apple Account. With the information provided it should be easy to identify which number you would have used when the Apple Account was set up.

If you are signed into that account on your iPhone, you can go to Settings > [name on top] > Password & Security, and the number is listed under Trusted Phone Numbers. If you currently do not have access to your account, you will not be able to view this number.

There are more ways to receive the code, have a look at this article:

Get a verification code and sign in with two-factor authentication - Apple Support